Method and device in power transistor

ABSTRACT

A method and a device for controlling a switching operation consisting of a turn on or a turn off operation in a voltage controlled power transistor is provided. At least one current source is arranged at the control electrode of the power transistor. The at least one current source controls the recharging of at least one of the capacitances which occurs between the control electrode of the power transistor and the main electrode of the power transistor to determine the time rate of change of at least one of the voltage and current.

This application is a divisional of U.S. patent application Ser. No.08/739,999 filed on Oct. 30, 1996 now U.S. Pat. No. 5,828,539.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to drives for power transistors, wherein the driveis used for controlling a power transistor during a turn-on, turn-offand current-limiting operation.

2. Description of Related Art

Self-commutated converters are presently used in a number ofapplications, for example in static converters for motor drives, supplydevices, UPS systems, etc. In such applications, an individual powertransistor is normally used for taking up the relatively high voltagesoccurring.

There are a number of types of power transistors, the most common beingMOSFET and IGBT transistors, bipolar transistors and Darlingtontransistors. The principles of drives disclosed through this descriptioncan be applied to all types of power transistors, but this descriptionis substantially directed to drives in connection withvoltage-controlled power transistors such as IGBTs. In the following,when referring to the electrodes of a power transistor, the designationsgate, emitter and collector are used, wherein the term emitterencompasses the word "source" used in MOSFET literature and,correspondingly, the term collector encompasses the word "drain".

The following description describes a few different reasons for thedesire to control the turn-on and turn-off operations in powertransistors. In this connection, the concept dv/dt control and theconcept di/dt control will be used. These concepts refer to methodswhich make it possible to control or limit the voltage derivative andthe current derivative, respectively, in connection with turn-on andturn-off of the transistor. The concepts turn-on and turn-off are usedas designations for switching the power transistor on and off,respectively.

During turn-on of a power transistor, for example an IGBT, the turn-onprocess is controlled by controlling the gate of the IGBT to preventoverload of a reverse-voltage diode (opposite diode) associated with theIGBT and caused by too fast current growth (di/dt) or too fast voltagebreakdown (dv/dt) across the power transistor. This is of particularlygreat importance for diodes of a higher voltage (≧1600 V). High-voltagediodes normally have a relatively large recovery charge due to arelatively high charge-carrier lifetime. This, in combination with a lowdoping level in the high-field region, makes the diode more sensitive todynamic avalanche or avalanche injection during turn-on, which may beharmful to the diode. This leads to a need to limit the amount of acurrent derivative (di/dt), which is negative during the turn-onoperation, and the voltage growth (dv/dt) to values which are acceptableto the diode. This can be done by turning on the opposite transistorsmoothly, that is, by maintaining the voltage derivative at the gatelow. At the same time, it is desired to keep the turn-on losses for thepower transistor and the turn-off losses for the diode as small aspossible.

During turn-off of the power transistor, control via the gate is used tocheck the voltage growth (dv/dt) for several different reasons. By meansof dv/dt control it is possible to limit the amplitude of that voltageovershoot which always occurs upon turn-off of a current in an inductivecircuit. This may be necessary to limit the stresses on the transistorin accordance with what is stated in the data sheets of the transistor.Control of the voltage derivative is also normally necessary uponturn-off of the power transistor when a short circuit or so-calledarc-through occurs (in connection with current limiting of the powertransistor). Without control or limitation of the voltage derivative(dv/dt), the transistor device can easily be damaged during turn-off ofa short-circuit pulse because of the fact that high peak voltagesotherwise easily arise.

When utilizing IGBTs of higher voltages (e.g. IGBTs in the voltagerange≧1600 V), there are, in addition, other difficulties. The SOA (SafeOperating Area) of the IGBT becomes dependent on the voltage derivativeat which turn-off occurs. By limiting the voltage derivative, highercurrents can be turned off or, alternatively, higher peak voltages canbe tolerated. This can be explained by the fact that dv/dt control,which is correctly performed, results in the electron injectioncontinuing for a considerable part of the turn-off operation, whichsuppresses the process which creates a dynamic avalanche such thathigher currents/voltages can be tolerated during the turnoff operationthan what would otherwise be the case.

Dv/dt control can also be used to limit dv/dt to which a load (e.g. amotor) is subjected, for a high dv/dt may entail local stresses on aninsulation in the load, which may successively break down theinsulation. Likewise, high voltage derivatives, dv/dt, may give rise tovoltage transients which are transmitted out on a cable, are reflectedand may generate voltage spikes which may result in insulation problems.High dv/dt values may also give rise to radio interference or disturbother electronic equipment. To fulfil the EMC standards (EMC=ElectromMagnetic Compatibility), it may be necessary to design filters whichreduce these disturbances. Dv/dt control may then be an aid in attackingthe problem, so to speak, at the source.

A negative factor to be considered is that control of the voltagederivative dv/dt normally increases the turn-on and turn-off losses to acertain extent. With full control of the turn-on and turn-offoperations, however, the losses may be minimized. There is always anoptimum way which the transistor can be turned on and turned off,respectively, if the aim is to minimize the turn-on and turn-off losses,respectively. One condition is that this optimum way is known and thatthe control parameters can be adapted such that the transistor is alwaysturned on/turned off in this optimum way.

There are also other reasons why it is desirable to control the turn-onand turn-off operations of a power transistor. One such case whereparticularly high demands are made on the control is during seriesconnection of transistors. During such series connection of powertransistors, where individual transistors are intended to take up partof a high voltage by voltage division, there are a number of factors toconsider. Some of the most important problems which have to be solvedare:

static voltage division;

dynamic voltage division; and

voltage division under short-circuit conditions.

Of these above-mentioned problems, the primary task of this descriptionis to seek a solution to the question of how dynamic voltage division isachieved in an optimum way, that is, during turn-on and turn-off of thetransistor. Various proposed methods for this are known. Among otherthings, there are several different methods whereby externalvoltage-dividing elements are used, for example a combination of adiode, a resistor, and a capacitor. These methods, however, do notresult in a solution to the problem at the source but only result in anattempt to limit the differences in voltages which arise acrossindividual transistor modules in a chain of series-connected transistorsto a level which may be tolerated, by adding external components. Thisincreases both the volume and the cost of, for example, a device in theform of a converter designed in accordance with such known principles.

In currently used converters, a very simple method of controlling orlimiting dv/dt and di/dt, respectively, is normally used. A common wayis that, during turn-on of the transistor, the gate is connected to avoltage source by means of a resistor, a so-called gate resistor. Thisresistor will limit the current delivered by the voltage source,whereby, by a suitable choice of resistor, it is possible to influencehow fast the turn-on occurs. In the same way, a combination of a voltagesource and another resistor may be used to influence how fast theturn-off is to occur. This method is simple and frequently used, but itonly provides limited control possibilities. During turn-on of thetransistor, it is not possible to influence di/dt and dv/dt,respectively, separately. Likewise, for example, the voltage derivativeduring turn-off is greatly dependent on at which current the turn-offoccurs (the higher the current, the greater the voltage derivative). Toavoid too high a voltage overshoot during turn-off of short-circuitcurrents, a method is often used in which it is first detected whetherthe on-state voltage of the transistor exceeds a given level. If this isthe case, the current is judged to be so high that turn-off must beperformed smoothly, that is, with a reduced voltage derivative, whichmeans that turn-off is carried out with a higher gate resistance thanwhat is normally the case.

The turn-off and turn-on operations at a given current are alsodependent on the temperature of the transistor. Further, largevariations in the turn-off and turn-on operations may occur whendifferent specimens of transistors of the same type are tested with thesame drive. Such variations may give problems with transient currentdivision in case of parallel connection of transistors and withtransient voltage division in case of series connection of transistors.Thus, there is a need of a method which in a better way can makepossible control of the turn-on and turn-off operations, respectively,of power transistors, partly with the aim of controlling the processbetter in its details to achieve a more optimum turn-on and turn-off,partly with the aim of reducing the dependence of the turn-on andturn-off operations on current, temperature and naturally occurringvariations of transistors of the same type.

Because the known solutions are not the optimum solutions to meetvarious requirements, the use of an "intelligent" drive, shown accordingto the invention, for control of a power transistor, such as an IGBT, inits so-called linear or controllable region during switching isproposed. The solution of the present invention is somewhat morecomplicated than the method which is most commonly used today involvingadaptation of the so-called gate resistors, but the costs incurredtherefor in many applications may be more than worthwhile since thepower transistors may be utilized more efficiently due to a more optimumcontrol.

SUMMARY OF THE INVENTION

The concept of the present invention is based partially on the fact thatthe gate-charge characteristic of a power transistor is known duringswitching of the transistor at different collector voltages and thatthis characteristic does not vary greatly between different specimens ofthe same power transistor. This is of special importance in a devicewhich comprises a number of power transistors cooperating in a circuit,for example in a high-voltage valve with series-connected powertransistors or in a high-current valve with parallel-connected powertransistors.

During switching of a power transistor, a certain charge has to be addedto (or removed from) the gate in order to cause the transistor to changebetween the on and off positions. By controlling the charging currentduring switching operations of the power transistor, it is possible tocontrol the switching-on and switching-off operations (here called theturn-on and turn-off operations, respectively). Attempting to controlthe charging current in such a method by using a voltage source and aresistor connected to the gate is not particularly advantageous. Themagnitude of the charging current and its time-dependence are thendependent on a number of factors, for example the leakage inductancebetween the gate and the power transistor, the internal gate resistancewhich is often applied by the manufacturer internally in the transistormodule, the threshold voltage of the transistor, on the crystaltemperature of the transistor, and the main current through the powertransistor.

A method, according to one aspect of the invention, is to supply to thegate a well controlled (e.g. current-source) charging current by meansof a controllable current source when the working point of thetransistor runs through the current-limiting (usually called the linear)part of the characteristic of the transistor. To obtain better controlof the collector voltage and the voltage derivative, dv/dt, it is alsoadvantageous to increase and linearize the Miller capacitance of thetransistor by introducing an extra capacitor between the collector andthe gate. This capacitor must generally be damped to preventoscillations.

The above described technique according to the invention offers manyadvantages for controlling transistors with an MOS gate, such as IGBTtransistors. Further, the technique permits control of the transistorduring switching and current-limiting operations and the object thereofis to minimize the switching losses, to increase the Safe Operating Area(SOA) or ensure in a reliable manner that the transistor is alwayslocated within SOA, to offer a reliable method for short-circuitprotection, to limit dv/dt, if desired, in a load, or to simplify EMCprotection, to provide good current division during parallel connectionof power transistors, and to provide good voltage division during seriesconnection of power transistors.

The technique is extremely suitable for controlling high-voltage IGBTs(1600 V and above), since a good control of the behavior of such atransistor during switching increases its SOA while at the same timeswitching losses can be minimized. However, the technique may also beused for other MOS-controlled power transistors, such as IGBTs andMOSFET transistors of lower voltage.

During turn-off of a power transistor, it may be of great importance tocontrol the voltage derivative dv/dt for different values of collectorvoltage, that is, that the voltage derivative with respect to the timemay be given different values for different collector voltages. At a lowcollector voltage, it is normally advantageous with a high value of thisvoltage derivative in order to keep the switching losses low. At a highcollector voltage, on the other hand (around nominal voltage orthereabove), it may be desirable to reduce the voltage growth, that is,to limit the voltage derivative to a lower value. A limitation of thevoltage derivative at this higher voltage will also increase what isusually designated SSOA (Switching Safe Operating Area), that is, thesafe operating area during switchings of the transistor. In addition,this may be used during series connection of transistors for the purposeof improving the dynamic voltage division. By limiting the voltagederivative to a very low value as soon as the voltage of the transistorexceeds a certain level, the maximum voltage picked up by the transistorin connection with a turn-off process may be limited.

According to another aspect of the invention, the voltage derivative iscontrolled to be dependent on the collector voltage in that a currentsource connected to the gate of the power transistor is controlled bythe collector voltage across it. A voltage divider is used to sense thecollector voltage, whereby the sensed voltage is used to control thevalue of the current delivered by the current source. This means thatthe drive where the variable current source is included is adapted tothe most suitable turn-off operation. If, for example, the transistor isturned off in one case where a current limitation is imposed on thedevice in which the transistor is included, that is, when a highcollector voltage occurs across the transistor even before the turn-offoperation is started, the drive of the gate can automatically sense thisand slowly turn-off the transistor (with a low voltage derivative) inorder to maintain the transistor within a safe operating area.

The capacitance with which the above-mentioned controlling currentsource cooperates may, in principle, consist of the self-capacitance ofthe power transistor between the gate and the emitter. To create abetter linearity and better control of the function, however, acapacitance is preferred which is substantially determined by acapacitor, connected in parallel with the self-capacitance of the powertransistor, between the gate and the collector.

The type of control described above is almost independent of the load.This means that full control of the turn-off operation is enjoyedindependently of the current through the load.

According to still another aspect of the invention, during turn-on ofthe power transistor, both the current derivative and the voltagederivative (relating to current and voltage, respectively, between themain electrodes of the transistor, the emitter and the collector), arecontrolled substantially independently of each other. The voltage of thegate is changed with the aid of a current source connected to thecontrol electrode in such a way that the gate-emitter capacitance ischarged. At a certain voltage level, V_(th), the power transistor startsto conduct. During the time of current growth (di/dt), the currentthrough the transistor is controlled by the voltage of the gate. Since,according to the invention, a capacitor which supplies an extracapacitance (in addition to the self-capacitance of the transistor) hasnow normally been connected between the gate and the emitter, thiscapacitor starts being charged as soon as the turn-on operation isinitiated. In order not to increase unnecessarily the turn-on delay andthe charge required for the turn-on operation, a Zener diode may beplaced in series with the capacitor, which Zener diode starts conductingwhen the voltage of the gate is approximately equal to V_(th). This isdone by selecting a Zener diode which starts conducting at a suitablevoltage. The total capacitance between the gate and the emitter (theexternal capacitor together with the self-capacitance of thetransistor), together with the magnitude of the gate current, determinesthe derivative of the voltage between the gate and the emitter, which inturn determines the current derivative (di/dt) across the transistor. Byselecting the value of the extra capacitor between the gate and theemitter, the current derivative may be reduced to the desired amount.During normal turn-on (no short circuit), the current derivative iscontrolled in the way mentioned until the collector current reaches thepeak value (i_(pk)) determined by the load and the opposite diode (seeFIG. 4). At this time, the voltage (v_(CE)) across the transistor startsfalling. Now, according to the invention, the voltage derivative dv/dtis instead controlled by means of the current source (intended for theturn-on operation) in combination with the capacitance between thecollector and the gate.

The current sources intended for turn-on and turn-off, respectively, arevoltage-limited, which means that the current therefrom drops towardszero when the voltage of the gate has reached the desired value forstatic on and off positions, respectively, of the controlled transistor.

In a series connection, for example, the invention makes it possible foran arbitrary number of power transistors to be series-connected in avalve, because the control device according to the invention is limitingfor a high voltage growth across individual transistors in the series.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, according to the prior art, a bridge connection with twoIGBTs and two opposite diodes, wherein the bridge connection mayconstitute a branch for a phase in a three-phase inverter.

FIG. 2 shows a conventional drive circuit for an IGBT, wherein thesetting of the turn-on operation and the turn-off operation is done byselecting the respective gate resistor to a value suitable for thetransistor and for the application.

FIG. 3 shows an example of an ideal control of the voltage derivativedv/dt during turn-off of an IGBT to avoid overvoltages during, forexample, turn-off of large currents in an inductive circuit or duringseries connection of a number of transistors.

FIG. 4 shows an example of control of di/dt and dv/dt independently ofeach other during turn-on of an IGBT to reduce the turn-on losseswithout exceeding the permissible amount for the negative currentderivative of the opposite diode.

FIG. 5 illustrates an embodiment for control of the power transistoraccording to the invention, wherein fixed values of di/dt and dv/dt forturn-on and dv/dt for turn-off have been selected in advance.

FIG. 6 illustrates an alternative embodiment for control of the powertransistor according to the invention, wherein the control of di/dt anddv/dt for turn-on and dv/dt for turn-off may also depend on thatcollector voltage which arises across the transistor prior to and duringsaid turn-on and turn-off operations, respectively.

FIG. 7 shows an example of the design of a voltage-limited currentsource which delivers a stable current with a low temperaturedependence, which is useful in those cases where it is desired tocontrol a power transistor in a very well-defined manner, which may bethe case in, for example, a series connection of a number of transistorsaccording to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

A number of embodiments of the invention will be described withreference to the accompanying drawings.

One typical field of use of the invention is during control of a bridgeconnection as shown in FIG. 1. This figure shows a prior art bridgeconnection with two power transistors in the form of two IGBTS (T1 andT2) and their opposite diodes (D2 and D1, respectively). Thedesignations used in the description are clear from the figure. A directvoltage V_(DC) is supplied to the bridge for conversion in two valves,each comprising a power transistor T1 and T2, respectively. The currentwhich flows through a power transistor, for example T1, is assigned thedesignation i_(c), whereas the current which flows through the oppositediode, for example D2, is designated i_(F). The voltage across atransistor is designated v_(CE). In the loop through theintermediate-link capacitor C_(DC), T1 and D2 (or C_(DC), T2 and D1),there is normally no intentional inductance, but an unavoidable strayinductance, here designated L1, influences the turn-on and turn-offoperations. The converted current out of the bridge is designated i_(ph)(i_(phase)). The drives for the transistors T1 and T2 have thedesignations DU1 and DU2, respectively.

FIG. 2 shows an example of a drive which, according to the conventionaltechnique, controls a power transistor, for example an IGBT. The controlis carried out such that a control unit GDC controls two switchessynchronously to alternately connect the gate G of the transistor to +15V and -5 V, respectively, via separate series resistors.

A drive which constitutes one embodiment of the invention is illustratedin FIG. 5. The designations C, G, E at the extreme right in the figurerelate to the collector, gate and emitter, respectively, of a powertransistor which is to be controlled with the drive according to thefigure.

According to the example, the emitter E is connected to the zero-voltageline of the drive. The gate G is connected by means of a first currentsource S1 to a driving voltage of +15 V. This first current source S1 iscontrolled to feed current to the contact line 1 of the gate G. Thecurrent from the current source S1 will thereby charge the capacitancewhich exists between the gate G and the emitter E. This capacitance maybe in the form of a self-capacitance between the above-mentionedelectrodes, but preferably a capacitor C1 is used, connected between thegate and the emitter, as complement to the self-capacitance in case itis desired to be able to freely set both di/dt and dv/dt largelyindependently of each other. In series with the capacitor C1 there is aZener diode Z1 between the emitter and the gate. In the simplestembodiment of the invention, this Zener diode is omitted, but in thatcase the current source S1 must deliver a somewhat larger charge.

The current source S1 is controlled by means of a gate drive controller(GDC) to deliver current while turning on the power transistor which iscontrolled by the drive, whereby the voltage rises on the contact lineof the gate. As mentioned, at a certain voltage level, V_(th), the powertransistor will then carry current. Depending on the choice of the valueof the Zener diode Z1, this Zener diode becomes conducting at a voltagewhich is approximately equal to V_(th). In this way, the capacitor C1starts to be charged in parallel with the internal capacitance of thetransistor between the gate and the emitter. Therefore, the capacitor C1will limit the rate at which the voltage vg grows between the gate G andthe emitter E, which in turn determines how rapidly the collectorcurrent of the transistor grows. The choice of the capacitance value ofthe capacitor C1 will thus determine the magnitude of the currentderivative during turn-on of the power transistor (see FIG. 4) when itis situated in its current-limiting (linear) working range, providedthat the transistor is turned on in a substantially inductive circuit.

After turn-on of the power transistor, a collector current i_(c) willflow between its main electrodes (C, E), whereby the current increasesto a peak value, i_(pk), which is determined by the load and theopposite diode. From now on, the current of the power transmitter is notcontrolled by the gate G. Instead, the voltage V_(CE) will drop and thevoltage derivative dv/dt, that is, the voltage breakdown across thepower transistor is controlled with the aid of the capacitance whichoccurs between the collector C and the gate G. According to theinvention, this capacitance could also, in principle, consist of aself-capacitance. However, also in this case, it is advantageous toswitch in an external capacitance between the collector and the gate inthe form of a capacitor C2. This capacitance linearizes the internalMiller capacitance, which makes the voltage derivative largelyproportional to the gate current (see FIG. 4). The value of thecapacitance of this capacitor C2 will then, in combination with themagnitude of the control current, be controlling with respect to themagnitude of the voltage derivative dv/dt.

In order to be able to control also a turn-off operation of the powertransistor in the desired way, the gate G, according to one embodimentof the invention, is further connected to a second current source S2,which communicates with a driving voltage of, for example, -5V. Thissecond current source S2 is controlled to feed current to the contactline 1 of the gate. The current source S2 then delivers a negativecurrent which recharges the capacitance which occurs between thecollector and gate of the transistor. Also during turn-off it ispossible to allow the capacitance between the collector and gate toconsist of the self-capacitance. However, to obtain better control ofthe turn-off operation, it is advantageous to switch an externalcapacitor C2 in parallel with the self-capacitance between the gate Gand the collector C.

During turn-off of the power transistor, the current source S2 iscontrolled by a gate drive controller GDC to deliver current duringturn-off of the power transistor which is controlled by the drive,resulting in the voltage on the contact line 1 of the gate dropping.This causes the current source to extract charges from the gate. If thepower transistor consists, for example, of an IGBT, the voltage of thegate drops to about +10 V before the transistor starts picking up avoltage V_(CE) >>V_(CEsat) (sat=saturation) across the main electrodes.The voltage on the gate at which this occurs is, however, greatlydependent on, inter alia, the current which flows through the transistorat this moment. During the initial stage of the voltage growth, thecurrent source will substantially charge the internal Miller capacitancebetween the gate and the collector. As the voltage increases, however,this capacitance decreases and, at a voltage of the order of magnitudeof 40 V, the external capacitor C2 will instead dominate. The voltagegrowth (dv/dt) will then be determined substantially by the current usedby the current source S2 and by the capacitance value of the externalcapacitor C2 (FIG. 3).

According to the above described operations, the capacitor C2 willdetermine the voltage derivative of the transistor in cooperation withthe current from the current source S1 during turn-on and the currentsource S2 during turn-off, respectively. In this way, the voltagederivatives during turn-on and turn-off may be chosen independently ofeach other. The size of the capacitor C2 is chosen such that itdominates over the Miller capacitance of the transistor at voltagesexceeding, for example, 50-100 V. The current that the current sourcesS1 and S2 need to deliver is then controlled by the maximum voltagederivative during turn-on and turn-off, respectively, which is desired.The current that the current source S2 delivers then normally becomes solarge that di/dt during turn-on becomes too large for an opposite diodeif only the internal input capacitance of the transistor (between thegate and the emitter) is allowed to determine this. Then, an externalcapacitor Cl may be added between the gate and the emitter to limit thecurrent derivative di/dt during turn-on to a value suitable for theopposite diode.

In this way, both di/dt and dv/dt during turn-on may be chosen largelyindependently of each other.

In order not to unnecessarily to increase the turn-on delay and thecharge which is required for the turn-on operation, a Zener diode Z1 maybe placed in series with the capacitor C1. During turn-off of the powertransistor, the Zener diode Z1 becomes conducting and the capacitor C1is charged to substantially the same voltage as the gate G of the powertransistor, for example -5 V. The breakdown voltage of the Zener diodeshould therefore, in this example, be chosen approximately equal to 5V+V_(th), such that the Zener diode during turn-on of the powertransistor starts conducting when the voltage of the gate G of thetransistor has reached the voltage V_(th). In this way, the capacitor C1influences the whole turn-on operation from the moment when currentstarts flowing between the main electrodes of the power transistor. By asuitable choice of the voltage level of the gate in the off-state of thepower transistor, it is also possible to prevent the capacitor C1 frominfluencing the turn-off operation of the power transistor. If, forexample, V_(th) is 5 V and the voltage level of the gate in the on-stateof the power transistor is +15 V, the voltage level of the gate in theoff-state of the power transistor should be ≦-5 V. A suitable breakdownvoltage for this Zener diode is then ≧10 V, and when the powertransistor is turned on, the capacitor mentioned has been recharged tomore than approximately V_(th), that is, +5 V. During the subsequentturn-off operation, the Zener diode Z1 will not, therefore, carrycurrent until the voltage between the gate and the emitter of the powertransistor falls below V_(th). Then the capacitor C1 can no longerinfluence the turn-off process of the transistor.

Preferably both current sources S1 and S2 are voltage-limited.

The primary task of the Zener diodes Z2 and Z3 is to limit the voltageof the gate such that the voltage does not exceed (e.g. in connectionwith a short-circuit) or fall below values specified for the transistor.

At least two ways of controlling the gate drive controller GDC can beused. The two current sources S1 and S2, respectively, can be controlledto deliver a current which varies according to a predetermined processduring the turn-on and turn-off operations, up to the time when thevoltage of the gate has risen or dropped to a voltage level which liesnear +15 V or -5 V, without the collector voltage v_(CE) being sensedand where the gate drive controller is only supplied with an inputsignal In which provides information about the times for switching onand off the transistor.

In another embodiment of the invention, a broadband voltage divider,comprising the RC circuits RC1 and RC2, is connected in parallel withthe emitter and collector connections of the power transistor (FIG. 6).Between these two RC circuits, a sensing line 2 for sensing thecollector voltage v_(CE) is then connected at the point 3. The voltageof this sensing line can then be used for controlling the gate drivecontroller GDC for increase of the current from the first and secondcurrent sources, respectively, to obtain the desired current and voltagederivatives according to the above during turn-on and turn-off,respectively. The gate drive controller GDC controls the current fromthe first S1 and the second S2 current source to be a predeterminedfunction of the collector voltage v_(CE). The dependence of the controlsignal on the collector voltage is thus described by the functioncs1(t)=f(v_(CE) (tau), tau≦t) as examples, where both instantaneousand/or historical values of v_(CE) may be utilized to influence thecontrol signal. This provides a possibility of creating, for example, avoltage derivative during turn-off according to FIG. 3, where thederivative is given different values step-by-step in dependence on thecollector voltage. As an example, FIG. 3 shows a value 4 kV/μs for thederivative initially during the voltage growth across the transistor,whereas the value of the derivative is reduced to 1 kV/μs when thevoltage approaches V_(DC). Values other than those shown may, of course,be chosen. Also, the decreasing value of the voltage derivative may becaused to undergo more steps than two, or be varied continuously.

The gate drive controller GDC delivers two control signals, one controlsignal cs1 for control of the current source S1 during turn-on and onecontrol signal cs2 for control of the current source S2 during turn-offof the power transistor. The gate drive controller GDC only utilizesknown technique and will not be described in more detail here. Thecontrol signals cs1 and cs2, respectively, may be both digital andanalog depending of the nature of the controlled current source.

Additionally, the current sources S1 and S2, respectively, are designedaccording to known techniques. Controllable current sources may bedesigned with ordinary transistor switches. If a very good temperaturestability is desirable, a switch according to FIG. 7 may be used. Thefigure illustrates an example of a current source S2 according to theinvention and constitutes in the exemplified case a modification of acurrent source of a kind which is marketed by, for example, SiliconixIncorporated. If a controllable current source with very good stabilityis desired, a larger number of digitally controlled current sources mayalso be used, the task of each of these current sources being to delivera fixed current. It is then possible to achieve a current source whichcan be rapidly changed between a number of different values of deliveredcurrent in order to realize the above-mentioned control where thevoltage derivative of the transistor can be given different values independence on the instantaneous value of the collector voltage.

The design of the current source S1 is the same as that described forS2.

cs₂ is, in the example according to FIG. 7, a digital signal whichcontrols the switch SW to change between two positions.

In the embodiments according to FIGS. 5 and 6, two controllable currentsources are shown. If only one of the turn-on or turn-off operationsneeds optimized control, it is, of course, possible to utilize only oneof the above-mentioned first and second current sources (S1, S2)together with the associated capacitor for control of the desiredderivative according to the above. In such a case, the second currentsource is replaced with a conventional solution comprising a voltagesource, a semiconductor switch and a resistor in accordance with FIG. 2.

We claim:
 1. A method for controlling a turn-on operation of a voltagecontrolled power transistor having a gate, a first main electrode and asecond main electrode, said method comprising the steps of:operating afirst current source to deliver current to the gate; and controlling arecharging of a capacitance between the gate and the first mainelectrode by means of the current delivered to the gate thus determiningthe time rate of change (di/dt) of the current (i) between the mainelectrodes of the power transistor during the turn-on operation;delivering a current with a predetermined course of time by means ofsaid current source; and controlling said current source by a controlsignal.
 2. A method for controlling a turn-on operation of a voltagecontrolled power transistor having a gate, a first main electrode and asecond main electrode, said method comprising the steps of:operating afirst current source to deliver current to the gate; and controlling arecharging of a capacitance between the gate and the second mainelectrode by means of the current delivered to the gate thus determiningthe time rate of change (dv/dt) of the voltage (v) across the mainelectrodes of the power transistor during the turn-on operation;delivering a current with a predetermined course of time by means ofsaid current source; and controlling said current source by a controlsignal.
 3. A method according to claim 2 further comprising the stepsof:delivering a second control signal to a second current source; andcontrolling said second current source by said second control signalduring turn-off of the power transistors such that a pre-determined rateof change of the voltage (dv/dt) during turn-off is obtained.
 4. Amethod for controlling a turn-off operation of a voltage controlledpower transistor having a gate, a first main electrode and a second mainelectrode, said method comprising the steps of:operating a secondcurrent source to deliver current from the gate; and controlling arecharging of a capacitance between the gate and the second mainelectrode by means of the current delivered from the gate thusdetermining the time rate of change (dv/dt) of the voltage (v) acrossthe main electrodes of the power transistor during the turn-offoperation; delivering a current with a predetermined course of time bymeans of said current source; and controlling said current source by acontrol signal.
 5. A method according to claim 4 further comprising thesteps of:delivering a second control signal to said second currentsource; and controlling said second current source by said secondcontrol signal during turn-off of the power transistors such that apre-determined rate of change of the voltage (dv/dt) during turn-off isobtained.
 6. A method for controlling a turn-on operation of a voltagecontrolled power transistor having a gate, a first main electrode and asecond main electrode, said method comprising the steps of:operating afirst current source to deliver current to the gate; and controlling arecharging of a capacitance between the gate and the first mainelectrode by means of the current delivered to the gate thus determiningthe time rate of change (di/dt) of the current (i) between the mainelectrodes of the power transistor during the turn-on operation;delivering a current which is dependent on the actual value of thevoltage (VCE) across the main electrodes by said current source; andcontrolling said current source by a control signal.
 7. A method forcontrolling a turn-on operation of a voltage controlled power transistorhaving a gate, a first main electrode and a second main electrode, saidmethod comprising the steps of:operating a first current source todeliver current to the gate; and controlling a recharging of acapacitance between the gate and the first main electrode by means ofthe current delivered to the gate thus determining the time rate ofchange (di/dt) of the current (i) between the main electrodes of thepower transistor during the turn-on operation delivering a current whichis dependent on the actual value and the historical values of thevoltage (VCE) across the main electrodes by said current source; andcontrolling said current source by a control signal.
 8. A methodaccording to claims 7, further comprising the step of:controlling saidcurrent source by an analog control signal.
 9. A method according toclaim 7, further comprising the steps of:composing a plurality ofdigitally controlled partial-current sources to form said currentsource, choosing the current from said current source by controllingeach partial-current source individually; and controlling eachpartial-current source individually by assigning a control signal toeach partial-current source.
 10. A method according to claim 9, further,comprising the steps of:delivering a first control signal from a gatedrive controller to said first current source; controlling said firstcurrent source by said first control signal to deliver a current duringturn-on of the power transistor for obtaining a predetermined time rateof change of at least one of a) the current (di/dt) through thetransistor, and b) the voltage (dv/dt) across the transistor.
 11. Amethod according to claim 9, further comprising the steps of:deliveringa second control signal from a gate drive controller to said secondcurrent source; controlling said second current source by said secondcontrol signal to deliver a current during turn-off of the powertransistor for obtaining a predetermined time rate of change of thevoltage (dv/dt) across the transistor.
 12. A method according to claim11, further comprising the step of:controlling the current from saidsecond current source by said second control signal to deliver a currentduring turn-off of the power transistor for obtaining decreasing valuesof the voltage derivative (dv/dt).
 13. A device for controlling aswitching operation in a voltage-controlled power transistor having acontrol electrode (G), a first main electrode (E) and a second mainelectrode (C) said device comprising:at least one current source coupledto the control electrode; a capacitance between the control electrodeand one of the main electrodes; wherein said at least one current sourcecontrols a recharging of said capacitance for controlling the timederivative of at least one from the group of: the derivative of thecurrent (di/dt) through the transistor during turn-on of the transistor,the derivative of the voltage (dv/dt) across the transistor duringturn-on of the transistor, the derivative of the voltage (dv/dt) acrossthe transistor during turn-off of the transistor; a second currentsource coupled to the control electrode (G) of the power transistor; anda second capacitance between the control electrode (G) and the secondmain electrode (C); and wherein said second current source controls arecharging of said second capacitance for determining the timederivative of the voltage (dv/dt) across the transistor during turn-offof the transistor.
 14. A device according to claim 13, furthercomprising:a first current source coupled to the control electrode (G)of the power transistor; and a second capacitance between the controlelectrode (G) and the second main electrode (C); and wherein said firstcurrent source controls a recharging of said second capacitance fordetermining the time derivative of the voltage (dv/dt) across thetransistor during turn-on of the transistor.
 15. A device according toclaim 13, further comprising:an external capacitor connected in parallelwith the control electrode and the main electrode, whereby saidcapacitance is the sum of the values of said capacitor and aself-capacitance between the control electrode (G) of the powertransistor and the main electrode.
 16. A device according to claim 13,further comprising:a plurality of digitally controlled partial-currentsources together forming at least one of said first and second currentsources.
 17. A device according to claim 13, further comprising:a firstcurrent source coupled to the control electrode (G) of the powertransistor; and a first capacitance between the control electrode (G)and the first main electrode (E); and wherein said first current sourcecontrols a recharging of said first capacitance for determining the timederivative of the current (di/dt) through the transistor during turn-onof the transistor.
 18. A device according to claim 17, furthercomprising:a gate drive controller (GDC) for generating control signalscontrolling said first and said second current sources to deliverpredetermined currents during the turn-on and turn- off operations,respectively.
 19. A device according to claim 18, further comprising:avoltage divider coupled between said first main electrode and saidsecond main electrode of the transistor for sensing the voltage acrossthe transistor and for generating a feedback signal being supplied tosaid gate drive controller, whereby the current from at least one ofsaid current sources is controlled by means of a control signal being afunction of said sensed voltage.
 20. A device according to claim 18,further comprising:a voltage divider coupled between said first mainelectrode and said second main electrode of the transistor for sensingthe voltage across the transistor and for generating a feedback signalbeing supplied to said gate drive controller, whereby the current fromat least one of said current sources is controlled by means of a controlsignal being a function of the instantaneous value and historical valuesof said sensed voltage.
 21. A method for controlling a turn-on operationof a voltage controlled power transistor having a gate, a first mainelectrode and a second main electrode, said method comprising the stepsof:operating a first current source to deliver current to the gate; andcontrolling a recharging of a capacitance between the gate and thesecond main electrode by means of the current delivered to the gate thusdetermining the time rate of change (dv/dt) of the voltage (v) acrossthe main electrodes of the power transistor during the turn-onoperation; delivering a current which is dependent on the actual valueof the voltage (VCE) across the main electrodes by said current source;and controlling said current source by a control signal.
 22. A methodfor controlling a turn-on operation of a voltage controlled powertransistor having a gate, a first main electrode and a second mainelectrode, said method comprising the steps of:operating a first currentsource to deliver current to the gate; and controlling a recharging of acapacitance between the gate and the second main electrode by means ofthe current delivered to the gate thus determining the time rate ofchange (dv/dt) of the voltage (v) across the main electrodes of thepower transistor during the turn-on operation; delivering a currentwhich is dependent on the actual value and the historical values of thevoltage (VCE) across the main electrodes by said current source; andcontrolling said current source by a control signal.
 23. A methodaccording to claims 22, further comprising the step of:controlling saidcurrent source by an analog control signal.
 24. A method according toclaim 22, further comprising the steps of:composing a plurality ofdigitally controlled partial-current sources to form said currentsource, choosing the current from said current source by controllingeach partial-current source individually; and controlling eachpartial-current source individually by assigning a control signal toeach partial-current source.
 25. A method according to claim 24,further, comprising the steps of:delivering a first control signal froma gate drive controller to said first current source; controlling saidfirst current source by said first control signal to deliver a currentduring turn-on of the power transistor for obtaining a predeterminedtime rate of change of at least one of a) the current (di/dt) throughthe transistor, and b) the voltage (dv/dt) across the transistor.
 26. Amethod according to claim 24, further comprising the steps of:deliveringa second control signal from a gate drive controller to a second currentsource; controlling said second current source by said second controlsignal to deliver a current during turn-off of the power transistor forobtaining a predetermined time rate of change of the voltage (dv/dt)across the transistor.
 27. A method according to claim 26, furthercomprising the step of:controlling the current from said second currentsource by said second control signal to deliver a current duringturn-off of the power transistor for obtaining decreasing values of thevoltage derivative (dv/dt).
 28. A method for controlling a turn-offoperation of a voltage controlled power transistor having a gate, afirst main electrode and a second main electrode, said method comprisingthe steps of:operating a second current source to deliver current fromthe gate; and controlling a recharging of a capacitance between the gateand the second main electrode by means of the current delivered from thegate thus determining the time rate of change (dv/dt) of the voltage (v)across the main electrodes of the power transistor during the turn-offoperation; delivering a current which is dependent on the actual valueof the voltage (VCE) across the main electrodes by said current source;and controlling said current source by a control signal.
 29. A methodfor controlling a turn-off operation of a voltage controlled powertransistor having a gate, a first main electrode and a second mainelectrode, said method comprising the steps of:operating a secondcurrent source to deliver current from the gate; and controlling arecharging of a capacitance between the gate and the second mainelectrode by means of the current delivered from the gate thusdetermining the time rate of change (dv/dt) of the voltage (v) acrossthe main electrodes of the power transistor during the turn-offoperation; delivering a current which is dependent on the actual valueand the historical values of the voltage (VCE) across the mainelectrodes by said current source; and controlling said current sourceby a control signal.
 30. A method according to claim 29, furthercomprising the steps of:composing a plurality of digitally controlledpartial-current sources to form said current source, choosing thecurrent from said current source by controlling each partial-currentsource individually; and controlling each partial-current sourceindividually by assigning a control signal to each partial-currentsource.
 31. A method according to claims 29, further comprising the stepof: controlling said current source by an analog control signal.
 32. Amethod according to claim 31, further, comprising the stepsof:delivering a first control signal from a gate drive controller tosaid first current source; controlling said first current source by saidfirst control signal to deliver a current during turn-on of the powertransistor for obtaining a predetermined time rate of change of at leastone of a) the current (di/dt) through the transistor, and b) the voltage(dv/dt) across the transistor.
 33. A method according to claim 31,further comprising the steps of:delivering a second control signal froma gate drive controller to said second current source; controlling saidsecond current source by said second control signal to deliver a currentduring turn-off of the power transistor for obtaining a predeterminedtime rate of change of the voltage (dv/dt) across the transistor.
 34. Amethod according to claim 33, further comprising the step of:controllingthe current from said second current source by said second controlsignal to deliver a current during turn-off of the power transistor forobtaining decreasing values of the voltage derivative (dv/dt).